The present invention is directed to a method for measuring the speed of an electrical machine having a stator, a rotor and a winding, in particular of an electronically commutated motor, a speed-measurement signal ω being sampled, in that a measured speed value ω(k) for the rotor is determined for each of a series of sampling instants.
A method of this kind for measuring the speed of an electronically commutated motor having a permanent magnet rotor, on whose periphery magnetic segments are arranged, which cooperate with an electronically commutated winding disposed on the stator, is known in the field. In this context, Hall-effect sensors mounted on the stator of the electronically commutated motor are used to record the position of the magnetic segments. Depending on the magnetic field, the output signal of the Hall-effect sensors is 0 or 1. Measured speed values are calculated from the time interval between two consecutive changes in the measurement signals of the Hall-effect sensors and from an angular offset between mutually adjacent magnetic segments in the circumferential direction of the rotor that is calculated from the number of pole pairs of the rotor. Since, with respect to their positioning on the periphery of the rotor and their dimensions in the circumferential direction, the magnetic segments exhibit manufacturing tolerances which are not known more precisely, deviations arise, however, between the actual angular offset and the calculated angular offset used in determining the measured speed values. Manufacturing tolerances of 20% are quite common. Given a constant rotor speed, such manufacturing tolerances result in the speed-measurement signal assuming, in rapid succession, arbitrary values in intervals from −20% to +20% of the actual speed value.
Another speed error can occur in response to a direction reversal of the rotor. During rotation in one direction, initially a first magnetic section and then a second magnetic section pull past a Hall-effect sensor. The resulting change in the magnetic field causes the output signal of the Hall-effect sensor to change. If the rotor comes to a stop and then moves in the opposite direction, the first magnetic section is then moved past the Hall-effect sensor again, thereby changing the output signal anew. The time interval between the two signal changes can be very short, which results in a very high speed being erroneously determined.
The speed signal could, in fact, be smoothed in that, for example, the mean value is generated in each case from the next-to-last, last and momentary measured speed value. However, the averaged measured speed value would then not reflect the speed at the current instant, but rather at the instant of the last change in the Hall-effect sensor signal. This time delay between the calculated speed signal and the actual speed signal is disadvantageous, above all when the speed signal is fed as an actual value signal to a speed control circuit. At low speeds, in particular, the delay time assumes relatively large values, so that the quality of the speed control decreases correspondingly.